1. Field of the Invention
The present invention relates to a magneto-optical recording medium used for recording and reproduction of information with laser light, utilizing the magneto-optical effect, and particularly to a magneto-optical recording medium and a magneto-optical recording method enabling high-speed recording of information on a recording medium.
2. Related Background Art
The conventional recording method using a magneto-optical recording medium require three processes (erasure of old data, recording of new data, and verification of whether the new data was correctly recorded or not) upon recording. Thus, the conventional methods need three rotations of the disk in order to rewrite information, and require much time to rewrite the information.
To solve this problem overwriting methods were developed (the optical modulation method and magnetic modulation method), which exclude the erasing process from the three step process noted above and which record new data directly on old data. Using the magnetic modulation method, it is difficult to raise the recording speed, because it is impossible to increase recording capacity by cementing two disks to each other because the magnetic field is applied from the opposite side to the light incidence side and because there is a limit on the magnetic field modulation speed of the magnetic head.
In contrast, the optical modulation method is a method for forming magnetic domains as modulating laser light to record information. This method permits the medium to have a large capacity in a double-sided, cemented structure and makes it easy to enhance the recording speed because the switching speed of the optical laser is higher than the magnetic modulation speed of a magnetic head.
For example, Japanese Laid-open Patent Application No. 62-1175948 describes attempts to use a magneto-optical recording medium provided with a recording layer in a double-layered structure comprised of, for example, a recording layer of TbFe and an auxiliary layer of TbFeCo, and to realize overwriting by first performing initialization and thereafter applying an external magnetic field and irradiating the medium with laser bemas of different powers.
In this method, recording is carried out as follows: magnetization of the auxiliary layer is preliminarily aligned in one direction by an initialization magnetic field prior to recording, a low-power laser is emitted to transfer magnetic information in the auxiliary layer to the memory layer so as to erase the information in the memory layer, a high-power laser beam is emitted to raise the temperature of the medium to a temperature higher than the Curie temperature (T.sub.c2) of the auxiliary layer, a recording magnetic field (opposite to the initialization magnetic field) is applied to invert the magnetization of the auxiliary layer, and the magnetization is transferred to the memory layer when the medium is cooled.
Further, Japanese Laid-open Patent Application No. 2-281442 discloses an attempt to use a magnetic phase transition material FeRh as an intermediate layer of an optical modulation overwriting medium to widen the laser power threshold range, thereby decreasing the initialization magnetic field. In this case, the FeRh intermediate layer is used in order to keep the magnetic interaction by electrostatic interaction between the recording layer and the auxiliary layer weak at room temperature but strong at a high temperature.
Further, Japanese Laid-open Patent Application No. 1-509020 discloses an attempt to perform optical modulation overwriting recording without the need to use the initialization magnetic field by providing the medium as described in above Japanese Laid-open Patent Application No. 62-175948 with an initialization layer.
However, the method of Japanese Laid-open Patent Application No. 62-175948 needs a strong initialization magnetic field in the double-layered structure of the magnetic layers as described above in order to invert the magnetization in the auxiliary layer at room temperature. Then, in order to decrease the necessary initialization magnetic filed to a practical level of strength, it is necessary to add the intermediate layer for decreasing the domain wall energy between the recording layer and the auxiliary layer, as described in Japanese Laid-open Patent Application No. 63-316343.
It is, however, necessary also in the case of the intermediate layer being added to apply a strong magnetic field of some kOe, which is also the case in the method as disclosed in Japanese Laid-open Patent Application No. 2-281442. This method needs an initialization magnetic field generator to be mounted in a magneto-optical recording apparatus, which makes reducing the size and cost of apparatus difficult.
Further, the method of Japanese Laid-open Patent Application No. 1-509020 practically needs five layers, an initialization layer, an auxiliary layer, a recording layer, an intermediate layer, and a switching layer, in order to perform stable recording and recording retention.
These methods thus need to use a recording medium in a thick structure as a whole, which needs some optical power upon recording. Then, the laser power becomes insufficient in recording at a high linear velocity, which disables recording.
The method of Japanese Laid-open Patent Application No. 1-509020, as described above, does not permit the so-called reflecting film structure, in which a reflecting film is provided to reflect incident light after transmission through the recording layer with efficiency and by which sufficient strength of the reproduction signal can be achieved also in the case of a magnetic body being arranged in a thin film, because the recording medium has the initialization layer.